set information about a property

I am making an editor that will look at javascript classes (es6), and display particular information about key properties. I wanted to try and use decorators to accomplish this, so something like this:

NOTE A Property Descriptor is not provided as an argument to a
property decorator due to how property decorators are initialized in
TypeScript. This is because there is currently no mechanism to
describe an instance property when defining members of a prototype,
and no way to observe or modify the initializer for a property. As
such, a property decorator can only be used to observe that a property
of a specific name has been declared for a class.

But then:

If the property decorator returns a value, it will be used as the
Property Descriptor for the member

So my understand was that while the function doesn't get the descriptor to change, it can "change it" by returning a descriptor like you did.
But I guess that this isn't the case as it doesn't work.

However, just below that it gives an example of how to do that using reflect-metadata: